Reproductive parameters of the fisher (Pekania pennanti) in the southern Sierra Nevada, California

Quantifying reproductive parameters is essential for developing effective conservation plans for species of concern; however, studying reproduction in wild settings can be challenging and local data are frequently unavailable. The fisher (Pekania pennanti) is an elusive forest-dwelling carnivore of conservation concern that occurs across much of boreal North America and reaches the southernmost limit of its distribution in the southern Sierra Nevada (SSN) in California. Data on fisher reproduction in this region are limited and applicability of parameters from other areas is uncertain. To clarify how fisher populations in the SSN compare with those elsewhere in North America, we conducted a comprehensive review of available literature on reproduction throughout the species’ range in North America. We then compared findings from the review with data we collected during 7 years in the SSN, focusing on the proportion of females reproducing, litter size, and the parturition date. On average across the fisher’s range, the proportion of adult females that reproduced was 0.71 (range: 0.40–1.00; n = 16 studies), litter size was 2.5 (range: 1–4; n = 16 studies), and parturition occurred on 25 March (range: 3 March–17 April; n = 16 studies). In our study area, we tracked 42 adult female fishers, 35 of which reproduced and used 257 reproductive dens (74 natal, 175 maternal, 8 early failures); 0.86 of our females attempted denning (range: 0.79–1.0; n = 93 opportunities) and 0.75 were successful (range: 0.64–1.0; n = 91 opportunities monitored through the den season). Mean litter size was 1.57 (range: 1–3; n = 75 litters) and mean parturition date was 30 March (range: 17 March–12 April; n = 69 natal dens initiated). Female fishers at the southern limit of their distribution in the SSN reproduced at a rate comparable to or higher than elsewhere in North America, but average litter size was the lowest reported for the species. Female fishers in the SSN gave birth at similar or later dates than elsewhere in their range. We explore factors that might explain patterns of variation in fisher reproductive parameters and discuss conservation implications of our findings.